The invention relates generally to improving range resolution of radar-illuminated targets. In particular, the invention enables the radar to detect and record data on a small target that could otherwise be obscured by larger targets.
The issue of range resolution has been has been an important part of radar design and research since the beginning of radar. Early designers of radar understood that using a short pulse for their radar allowed for separating closely spaced targets better than long pulses. However, imparting sufficient energy on the target to ensure its detection required radar pulses be of a certain minimum length, determined by the radar range equation and transmitter peak power. Hence, the requirement to achieve energy on target often trumped the requirement to distinguish multiple targets.
The first major advance in the area of range resolution occurred with the development of pulse compression. Pulse compression is achieved by modulating the radar pulse and then processing it with a matched filter on receiver. For proper modulation of the transmitted pulse, the response of the matched filter compresses the pulse to a width that can be reduced by the time-bandwidth product of the modulated pulse. This enables detection of two identical amplitude targets that can be spaced closer by the time-bandwidth product of the modulated pulse. Good modulation produces small sidelobes.
FIG. 1 shows a graphical view 100 of a pulse received signal signal. The abscissa 110 represents time or range, and the ordinate 120 represents signal strength or amplitude. At the origin where time and range are zero, the amplitude reaches maximum extent denoted as an un-modulated pulse 130 orthogonal to the span 140.
A matched filter response 150 provides a linear rise across the span's extent. A pulse-compression matched filter response 160 to the modulated pulse exhibits a narrower extent across the span 140 than for the un-modulated, with sidelobes 170 disposed adjacent thereto. However, sidelobes can interfere with the detection of small targets in the presence of large targets.
FIG. 2 shows a graphical view 200 of a secondary signal. The abscissa 210 represents time or range, and the ordinate 220 represents signal strength or amplitude. The pulse-compression matched filter response 160 can be compared against a second target, which from compression includes a detectable response 230 and anon-detectable response 240 that remains obscured by the sidelobes 170.
Under these circumstances, one can observe that a closely spaced target of equal amplitude can easily be detected. However, a target whose amplitude is no larger than the sidelobes cannot be reliably detected because the sidelobes interfere. Therefore, pulse compression sidelobes can produce significant limitations on the ability to detect or distinguish two closely spaced targets when one of the targets has significantly smaller amplitude than the other target.